Objects molded of synthetic resin are usually formed in a mold cavity defined between two mold halves. The halves are closed together and the synthetic-resin material is injected into this cavity where it allowed to harden.
Two principal methods exist to eject the hardened or semihardened workpiece from the mold cavity. The most common method simply mechanically punches this workpiece out. Such an arrangement is disadvantageous in that it frequently damages the workpiece, especially when made of a synthetic resin such as polyurethane which does not harden rapidly. An attempt has been made to avoid this disadvantage by providing a punch of mushroom shape, having an enlarged head which is used to press the workpiece out of the mold cavity. Such an arrangement has the considerable disadvantage that unhardened synthetic-resin material frequently finds its way underneath the enlarged head of the punch and thereby prevents it from being fully retracted afterward. Thus the device must be disassembled and cleaned thoroughly.
Another known method of ejecting a synthetic-resin workpiece from a mold cavity uses gas pressure. With such an arrangement an orifice is formed within the inner wall of the mold cavity and is connected to a source of pressurized gas. After the molding operation is completed this gas is forced through the orifice into the mold chamber beneath the hardened or partially hardened workpiece, thereby pneumatically pressing the workpiece out of the mold cavity. This system is not usable with synthetic resins which are very thin when in a fluid state. Polyurethane for instance in its liquid form frequently can enter gaps and orifices having maximum open widths of 1 micron so that when such a synthetic resin is used the orifice is plugged after a few cycles. In addition the gas as it enters through the small orifice frequently damages the still-soft molded article. Yet another disadvantage of this pneumatic workpiece ejection is that the orifice provided in the mold cavity for admission of the gas leaves a bump on the surface of the molded article, a disadvantage which frequently makes the workpiece unusable.